The invention relates to a two to four-wire hybrid circuit for coupling a bidirectional terminal line to a transmission medium having separate branches for a receiving and a transmitting direction of analog electrical signals.
Hybrid circuit arrangements for telecommunication systems are very well known in the art. Such a circuit arrangement couples usually a subscriber set or more generally the two-wire terminal line to a four-wire transmission medium, usually a transmission line. A main element of a conventional hybrid circuit arrangement is a coupling circuit designed as transformer which has to transform output voice signals from the two-wire terminal line to the transmitting branch of the four-wire transmission line or to couple input voice signals from the receiving branch of the four-wire transmission line to the two-wire terminal line. The principle object of such a hybrid circuit arrangement is to provide adequate transmission characteristics in both signalling directions with minimal loss while having a high cross-talk attenuation from the receiving branch to the transmitting branch of the four-wire transmission line.
A main problem in designing an appropriate hybrid circuit is that a non-real line impedance is present on the two-wire side. Well-known hybrid circuits have a balancing network composed of a frequency-dependent attenuator to compensate this influence. This balancing network, which comprises among other things a relatively high additional inductance, is arranged on the four-wire side of the hybrid circuit. Analog signals coming in on the receiving branch of the transmission line are restored by a receiver amplifier, whereas the outgoing signals are fed to inputs of a transmitter amplifier. It depends upon the point of view whether these amplifying stages are considered to be parts of the hybrid circuit arrangement or the transmission medium. In any case, however, the interfacing characteristics of the amplifier stage, the output impedance, the input impedance, respectively, have to be taken into consideration for designing a proper hybrid circuit.
Since conventional hybrid circuits have constraints in view of electrical characteristics, such as cross-talk attenuation and the mechanical design, such as space requirements, efforts have been made also for designing so-called active hybrid circuits. A known active hybrid circuit is described in an article "Line Circuit Interfaces For Additional Switching Systems" by G. Dooley in the IEEE Transactions On Communications, Vol. Com-27, July 1979, pages 978 to 981. The disclosed hybrid circuit is composed of a thick-film microcircuit comprising a receiver amplifier and a transmitter amplifier connected to the receiving branch and to the transmitting branch, respectively. A balancing network is arranged between the output of the receiver amplifier and the input of the transmitter amplifier. Symmetrically to the two-wire terminal line there is arranged a pair of battery feed arrangements, each connecting a pole of the supply voltage to a respective wire of the terminal line. Each battery feed arrangement comprises a series connection of a switching transistor and a battery feed resistor. Each switching transistor is controlled by an output signal of a respective one of the second amplifiers. Inputs of these amplifiers, in turn, are connected to a respective one of two outputs of the receiver amplifier, which outputs, deliver output signals in phase opposition.
In order to transmit voice signals in opposite direction from the terminal line to the transmission line, there is arranged a third amplifier which is connected by inputs to the terminal line and by an output to the transmitter amplifier. The battery feed resistors and the balance network are still implemented using discrete components. The thick-film hybrid circuit allows precision laser trimming of those resistors which require absolute or ratio trimming which is still believed to be necessary for maintaining the required trans-hybrid loss and common mode rejection.
This example of an active hybrid circuit exposes clearly the difficulties involved with implementing such a hybrid circuit notwithstanding its simple structure. Difficulties will further increase if another objective is taken into consideration. Presently available telecommunication systems and even more advanced systems have to be easily adjustable to different applications in different countries for matching different national standards. This means specifically in view of a hybrid circuit that provisions have to be made allowing adjustments to different national standards with respect to characteristics of either the terminal line or the transmission line. It may be obtained from the afore-mentioned characterization that such standards, of different line impedances for example, have to be carefully matched by an implemented hybrid circuit. It is believed that this object at least presently cannot be reached by a singly fully integrated active hybrid circuit. A design of different hybrid circuits for each application on the other hand is also not acceptable in view of design and manufacturing costs.
It is, therefore, a general object of the present invention to provide an improved two- to four-wire hybrid circuit of a telecommunication system with high cross-talk attenuation.
Another object of the present invention is to provide a hybrid circuit designed for moderate adjustments to different impedances, especially terminal line impedances with low cost and minimal impact on either equipment on the side of the terminal line or the design of the transmission medium.